Jeremy Corbyn faces a battle over Labour’s Brexit policy later as members choose between two competing strategies at the party’s annual conference.
They will decide whether Labour should explicitly back Remain in any future referendum, or adopt Mr Corbyn’s stance of staying neutral until a later date.
Unison has broken ranks with other unions to back the Remain motion.
The BBC’s Laura Kuenssberg said this could “shift the balance” and increase the chance of Mr Corbyn being defeated.
With unions controlling half of the votes at the conference, their position is vital.
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The majority of Labour’s 12 affiliated unions are expected to back the leadership after agreeing a joint position with Mr Corbyn two weeks ago.
But our political editor said Unison’s move was very significant, and makes it more likely that conference decides to back a Remain stance.
Labour’s position on Brexit has dominated the conference agenda, with huge disagreements over the issue.
The party’s draft plan for its Brexit policy, put forward by Mr Corbyn, suggests that, if Labour wins power in a general election, it would remain neutral while negotiating a new deal with the EU within three months.
It would then hold a referendum within six months, and the party would decide which side to back ahead of that at a special conference.
But grassroots activists at the conference have campaigned for an unambiguous Remain stance.
Late on Sunday, attempts to find a consensus during a behind-closed-doors “compositing” meeting failed to agree on words for a single Brexit motion.
As a result, members will debate, then vote, on three separate motions on Monday, including one calling on Labour to campaign “energetically” for Remain.
Speaking on BBC Radio 4’s Today programme, John McDonnell said he would support Remain in any future vote as he thought it was the “best option”, but he believed other members should be allowed to “exercise their own judgement”.
The shadow chancellor added: “This is an honest, democratic debate and that is what our party is about – making sure that people can express their views democratically and be honest about their assessments.
“People have high emotions on this because they feel it is important, but people are respecting each others views as well.
“Do not mistake democracy for division. It isn’t. What we are seeing is an honest debate.”
Other senior figures in Labour like shadow Brexit secretary Sir Keir Starmer and shadow foreign secretary Emily Thornberry – due to address delegates on Monday – also support a full-throated Remain stance.
Hilary Benn, Labour MP and chair of the Commons Brexit committee, told BBC Radio 4’s Today programme Labour’s position of a second referendum was “sensible” and the “the only way out of the deadlock we are facing”.
But he said it was clear the bulk of the party – and the shadow cabinet – had already confirmed they would back Remain.
It is going to be a very bumpy day. This afternoon the Labour Party, in all its glory, will debate the two positions of Leave and Remain.
One motion is overtly, full-throated, “Labour must campaign now to stay in the European Union.”
The other, the leadership position that was agreed in a small meeting with the unions some weeks ago, is to wait and see.
Now, of course, there is a logic in that position, trying to straddle both sides of the debate and trying to have an offer for people who voted for Brexit.
But there is such frustration about that here at conference – you can hear it boiling over in the bars and in every single fringe event.
The reason why it matters is not just that Labour has found it hard making its mind up about Brexit, but also we might be just a couple of months from a general election.
Labour is being radical this week, putting forward lots of ideas like free personal care, making it easier for people to buy electric cars and abolishing prescription charges.
But while people here are arguing about Brexit, the idea that comes from the conference to the country is about a party at odds with itself, fighting against itself, not a government in waiting.
Andrew Lewin, of Remain Labour, said the prospect of alternatives being put to a vote was “a huge step forward”.
“Labour members will have the chance to change party policy – so Labour commits to campaign to Remain in all circumstances,” he said.
Michael Chessum, of left-leaning group Another Europe, told the BBC that Monday’s vote was not just about Brexit.
“It’s about where Mr Corbyn and the unions are really willing to listen to party members,” he said.
Speaking to the BBC’s Andrew Marr show on Sunday, Mr Corbyn would not be drawn on which side he would back, saying “let’s see” what kind of new deal he was able to negotiate with the EU.
He said the party must respect the result of the Brexit referendum and do more to understand why people voted to leave.
Away from Brexit, Labour has announced a pledge to introduce free personal care in England for over-65s most in need of it, so they will not have to pay for help with dressing, washing and meals.
At the moment, those with savings of more than £14,250 have to contribute to the cost of home or residential help.
It is the latest of several new policies announced at the conference, including pledges to:
- Get rid of Universal Credit
- Scrap Ofsted, which it called “unfit for purpose”, and replace it with a new schools inspections system
- Make NHS prescriptions free in England, bringing it in line with Scotland, Wales and Northern Ireland
- Force large employers to provide flexible working hours to women experiencing symptoms of the menopause
On Sunday, party members also voted in favour of integrating private schools into the state sector.
Independent schools would have their charitable status withdrawn, as well as other public subsidies and tax privileges, and universities would have to admit the same proportion of private school students as in the wider population, currently 7%.
The vote means members have signalled a desire for the policy to be included in Labour’s next general election manifesto.
The start of the conference, on Saturday, was overshadowed by a failed attempt to oust deputy leader Tom Watson.
Mr Watson later said he was disappointed by the move but called for unity.
And on Sunday, one of Mr Corbyn’s senior aides, Andrew Fisher, announced his intention to resign as head of policy.
Mr Corbyn confirmed that Mr Fisher was leaving, saying it was a “very stressful and full-on job”.